It is well known that polymerisation conditions can be adjusted to produce a wide variety of products. This is also true for the production of ethylene copolymers. It is not unusual that one reactor system can produce resins useful in injection moulding, blow moulding, rotomoulding applications, wire coating, piping and films. Fluidised bed technology can also be used to make a wide variety of polyolefin products, e.g., homopolymers and copolymers of polyethylene, polypropylene, C4-C12 alpha olefins; ethylene-propylene-diene monomer (EPDM), polybutadiene, polyisoprene, and other rubbers.
However, generally, the polymer products made by a given reactor system use the same reactants but in different ratios and at different temperatures. Each of these polymer products can be made with a number of different resin properties, or grades. Each grade of polymer product has a narrow limit on its properties, e.g., density and melt index. Industrial reactors require time to adjust to the new conditions (e.g., temperature, reactant pressures, and reactant ratios) and produce material in the interim that is constantly changing but not within the properties (e.g., melt index and density) of either the old product or the new one. New products cannot be made instantaneously and require a quantifiable period of transiency in becoming adjusted to the new, desired conditions.
Generally, industrial control systems for gas phase, fluidised bed polymerisation reactors are designed to permit the operators to control the reactor by allowing the operators to select a desired melt index and density. Correlations of these properties are usually well known by the operators and those in the art for the particular reactor design and catalyst used.
The prior art has devised a number of methods to reduce the transient, off grade material. These methods typically involve some combination of adjusting the automatic flow/ratio controllers to a new value either at or above the ultimately desired value (“dial-in transition” and “overshoot”), removing the reactant gas entirely (“inventory blow down”), reducing the level of the catalyst (“low bed”), and adding a nonreactive gas (“nitrogen addition”).
DE 4,241,530 describes using a kill gas to stop a polymerization reaction, blowing the gas inventory far that reaction out of the reactor, and rebuilding a new gas inventory for a new product. This method reduces transition material. The cost associated with throwing away the old gas inventory and rebuilding a new inventory is very high for commercial transitions between closely related grades.
The prior art also discloses additional discontinuous transition process, said process usually including a gas phase purge and the addition of catalyst killer compounds.
McAuley et al. (“Optimal. Grade Transitions in a Gas Phase Polyethylene Reactor”, AIChE J., Vol. 38, No. 10: 1992, pp. 1564-1576) discloses three manual, labour-intensive transition strategies for gas phase polyethylene reactors. The first is an adjustment to the controls to overshoot the melt index and density values. The hydrogen feed and co-monomer feeds are increased to meet the designated properties. The second is an increase in temperature and manipulation of the slow vent to move the melt index of the produced product. The third is a drop in the catalyst level while keeping the bed resin residence time at a constant value to reduce off grade production.
Debling, et al., “Dynamic Modeling of Product Grade Transitions for Olefin Polymerization Processes”, AIChE J., vol. 40, no. 3:1994, pp. 506-520) compares transition performance of different types of polyethylene reactors. The article discloses seven separate manual, labour intensive transition strategies: (1) dialling in the final aim transition; (2) gas inventory blow down and simple dial-in transition; (3) low bed and simple dial-in transition; (4) gas inventory blow down and overshoot of melt index and density transition; (5) low bed, gas inventory blow down, and overshoot transition; (6) low bed and overshoot transition; and (7) gas inventory blow down, overshoot, and nitrogen addition transition.
EP798318 claims a process for controlling a gas phase polymerization reaction in a reactor when changing from a first product made at a first set of conditions to a second product made at a second set of conditions, said process comprising the steps of    (a) comparing the first product reaction temperature and the second product reaction temperature, change the product reaction temperature setpoint to the second product reaction temperature if said second product reaction temperature is lower than said first product reaction temperature,    (b) setting a melt index setpoint that is either 0-150% higher or 0-70% lower than the desired second product melt index value,    (c) setting a reaction temperature setpoint that is: 1-15 DEG C. above the desired second product reaction temperature if the second product melt index value is higher than the first product melt index value, or 1-15 DEG C. below the actual second product reaction temperature if the second product melt index is lower than the first product melt index,    (d) setting a product rate-limiting reactant partial pressure setpoint that is: 1-25 psig either below the first product rate-limiting reactant partial pressure if the second product melt index value is higher than the first product melt index value, or above the first product rate-limiting reactant partial pressure if the second product melt index value is lower than the first product melt index value;    (e) maintaining said melt index setpoint, temperature setpoint, and rate-limiting reactant partial pressure setpoint values until said polymerization product exhibits an average melt index and average product density with an acceptable range from the desired second product melt index value and second product density value;    (f) changing said melt index setpoint to the desired second product melt index value;    (g) changing said product reaction temperature setpoint to a value that is: (i) 0-15 DEG C. above said desired second product reaction temperature if the second product melt index value is higher than the first product melt index value, or (ii) 0-15 DEG C. below said desired second product reaction temperature if the second product melt index value is lower than the first product melt index value;    (h) changing said rate-limiting partial pressure setpoint to a value that is: (i) 0-25 psig either below the desired second product rate-limiting partial pressure if the second melt index value is higher than the first melt index value, or (ii) 0-25 psig above the second product rate-limiting partial pressure if the second melt index value is lower than the first melt index value; and    (i) changing the reaction temperature setpoint and the rate-limiting reactant partial pressure setpoint values to the desired second product reaction temperature and second rate-limiting reactant partial pressure value when the reaction product exhibits an average melt index value within acceptable limits of the second product melt index value.
EP798318 depicts in its FIGS. 3-5 a flowchart of the process control. The initial steps are similar to FIGS. 1-2 (which depict the EP798318) prior art methods, i.e. a transition including the lowering of the bed level.
All the examples of EP798318 relate to transitions between ethylene copolymers having the same comonomer, i.e. hexene.
EP1578808 relates to processes for transitioning among polymerization catalyst systems, preferably catalyst systems, which are incompatible with each other. In particular, it relates to processes for transitioning among olefin polymerization reactions utilizing Ziegler-Natta catalyst systems, metallocene catalyst systems and chromium-based catalyst systems.
Again, this prior art includes inter alia the step of lowering the reactor bed during the transition; and the examples relate to transitions between ethylene copolymers having the same comonomer, i.e. hexene.